The X-ray CT apparatus is a device for irradiates an examinee with a fan-beam (sectorial beam) or cone beam (conical or pyramidal beam) type X-ray to obtain projection data as information concerning the intensity of the X-ray transmitted through the examinee, and images distribution information of an X-ray absorption coefficient of the inside of the examinee on the basis of the projection data.
The X-ray CT apparatus obtains projection data at a discrete X-ray tube position (view). The number of views per rotation of an X-ray tube normally ranges from several hundreds to several thousands. The X-ray CT apparatus irradiates the examinee with X-ray while rotating the X-ray tube around the examinee, thereby performing scanning and obtaining projection data of views whose number is required to reconstruct one tomogram (CT image).
When X-ray tube current is constant as a scan condition of the X-ray CT apparatus, overabundance or deficiency of the exposed dose may occur in accordance with an X-ray irradiation angle or irradiation site. In connection with this, an X-ray CT apparatus for controlling X-ray tube current on the basis of scanogram projection data to lessen the exposed dose and enhance the image quality has been proposed (for example, see “Patent Document 1”).
Furthermore, an X-ray CT apparatus for calculating an elliptical cross-sectional model of an examinee from the scanogram projection data and calculating an X-ray tube current value from the area and horizontal to vertical ratio of the elliptical cross-section has been proposed (for example, see Patent Document 2”).